Broadly speaking, aerosol spray devices comprise a container holding a liquid to be discharged together and an outlet nozzle associated with a valving arrangement which is selectively operable to allow discharge of the liquid as a spray from the nozzle by means of the propellant provided within the container.
Both “compressed gas propellant aerosols” and “liquefied gas propellant aerosols” are known. The former incorporate a propellant which is a gas at 25° C. and at a pressure of at least 50 bar (e.g. nitrogen, carbon dioxide or air). Such a gas does not liquefy in the aerosol spray device. On opening of the valving arrangement, the compressed gas “pushes” liquid in the spray device through the aforementioned nozzle that provides for atomisation. There are, in fact, two types of “compressed gas propellant aerosols”. In one type, only liquid from the container (“pushed-out” by the compressed gas) is supplied to the outlet nozzle. In the other principal type, a portion of the propellant gas from the container is bled into the liquid being supplied to the nozzle which atomises the resulting two-phase, bubble-laden (“bubbly”) flow to produce the spray. This latter format can produce finer sprays than the former.
In contrast, “liquefied gas propellant aerosols” use a propellant which is present (in the aerosol spray device) both in the gaseous and liquid phases and is miscible with the latter. The propellant may, for example, be butane, propane or a mixture thereof. On discharge, the gas phase propellant “propels” the liquid in container (including dissolved, liquid phase propellant through the nozzle).
It is well known that “liquefied gas propellant aerosols” are capable of producing finer sprays than “compressed gas propellant aerosols”. This is due to the fact that, in the former, a large proportion of the liquefied gas “flash vaporises” during discharge of liquid from the aerosol spray device and this rapid expansion gives rise to a fine spray. Such fine sprays cannot generally be achieved with “compressed gas propellant aerosols”, in either of the two principal formats described above.
Attempts have been made to improve the “fineness” of sprays generated by “compressed gas propellant aerosols”. Prior art proposals have included the possibility of “bleeding off” some of the compressed gas (e.g. nitrogen) that is present in the container and mixing this with the liquid product to achieve “two fluid atomisation” which is a technique known to provide fine sprays for other areas of spray technology, e.g. liquid fuel combustion. However it has been found extremely difficult to produce fine sprays using two fluid atomisation with aerosol spray devices, and the nearest approach has been to use the equivalent of a vapour phase tap (VPTs are used in “liquefied gas propellant aerosols”) to bleed some gas into the valve. However results for improving spray fineness have not been significantly beneficial.
It is therefore an object of the present invention to obviate or mitigate the above mentioned disadvantages.